1. Field of the Invention
The present invention relates to a method of and a circuit for Ni-Cd battery charge control, for batteries which are used on low earth orbit spacecraft missions.
2. Discussion of the Background
Orbit time for such missions being typically about 1.5 hours of which about 1 hour in sunlight, during which period the battery needs to be charged, and about 0.5 hour in eclipse, during which period the battery is discharged, the battery charge and discharge rates are very high: for example, the batteries can experience up to 6000 discharge/charge cycles per year exclusive of peak load demands.
High charge currents and fast recharge require moderately complex electronics for controlling overcharge and for preventing undesirable battery heating.
With regard to battery overheating, it is difficult to correctly specify simple temperature-sensitive controls which would ensure energy balance, provide overcharge protection and control thermal dissipation throughout a mission wth changing environment and battery I-V characteristics.
Existing methods of battery charge control for spacecraft are outlined below:
current-limited or constant-current charging is a technique according to which a regulator limits charge current to one or more selected values. Charge termination is based on the measurement of battery voltage. If this voltage reaches a value corresponding to a defined recharge ratio, charging is switched to a lower current limit; PA1 voltage-limited charging is a technique according to which the current rate is controlled indirectly during overcharge. A regulator limits battery-charge voltage so as to reduce charge current from an initial value to a lower value that corresponds to a taper charge rate. Here again, charge termination depends on the reaching of a battery voltage limit; PA1 pressure-limited charging is based on the fact that oxygen is developed at an increasing rate as the point of full charge is approached and, depending on the rate of simultaneous recombination of the gas, a net pressure increase will occur within the cell. The rise in pressure can be used to generate a signal for charge control. A disadvantage of this method is that it required cell modification in order to sense the pressure by gages or by an auxiliary electrode. The reliability of this method has, so far, not proved to be very high; PA1 temperature-limited charging is based on the fact that the heat rate of a battery increases as it nears the fully charged state at constant current. Battery temperature is thus a useful alternative to battery voltage for use as a signal that overcharge is commencing. The effectiveness of the method depends on the thermophysical properties of the cell and the particular battery-system design application. With short cycles and high rate charging and a temperature sensor attached to the cell container cover, there is not sufficient time for the electrode stack to absorb the heat produced and to exhibit a temperature rise before the end of charge period. Therefore, application of this method has been limited to geosynchronous missions where time scales are more favourable in view of the long-period orbits. A problem with temperature-limited charging is that battery tmperature may also increase due to external heat inputs, and not only due to charging. PA1 in that the battery charge termination is established in a precise way; PA1 in that battery behaviour is predictable; PA1 in that approximately 30% less solar array charging power is required in comparison with the known methods; PA1 in that the battery cycle life is longer than that corresponding to the known techniques, since the battery degrades linearly during only the first few hundred cycles until steady state has been reached, after which no further degradation is observed; PA1 in that the battery charge termination is virtually independent of the environmental temperature fluctuations, which makes the battery cell temperature measurments according to the present invention more reliable than those described in the prior art.
The charging-techniques as described above, which are all based on constant current charge up to a defined end of charge voltage level, which is battery temperature dependent, followed by a gradually reducing current keeping the battery voltage at a defined end of charge voltage level for the remainder of the charging time, have drawbacks when applied to low earth orbit missions. Tests have shown that battery behavior is unpredictable in low earth orbit applications, whihc is believed to be mainly caused by incorrect battery charge termination associated with the known charging techniques. Another problem is the waste of solar array charging power due to the charging techniques used, requiring 30% more solar array power than the average power theoretically required. This is because the battery charging current is not constant during the complete charging period, but is reduced upon the reaching of a defined battery voltage limit or upon a rise in battery pressure or temperature